In recent years, in the field of electronic devices, there have been rapid developments of compact and portable electronic devices represented by mobile phones and personal digital assistants according to technical advances. Under the circumstances, there is a need for batteries for such electronic devices which batteries are compact and usable for a long time. As for such batteries, there is an industry-wide shift from conventional single-use primary batteries to secondary batteries, which can be charged and discharged repeatedly. An example of the secondary batteries is a nickel hydrogen secondary battery, and the battery has been developed to achieve higher performance. Another secondary battery, a lithium ion secondary battery, which has even higher performance, has also been developed and certain performance has already been obtained. The lithium ion secondary battery is nearing widespread commercialization. However, portable electronic devices are dramatically gaining more functions and higher performance, thereby increasing the power consumption of the devices. Thus a high performance battery is required that can ensure sufficiently long hours of continuous operation for portable electronic devices. Under these circumstances, a fuel cell receives attention as a next generation battery replacing the secondary batteries.
Fuel cells using hydrogen gas as their fuel generally provide high power densities. Thus a polymer electrolyte fuel cell (PEFC) using hydrogen fuel has been developed as a power supply for a high-speed movable body such as an electric vehicle or as a distributed power supply for homes, offices, stores, or the like.
Typically, the PEFC is formed by stacking a plurality of cell units to configure a stack thereby increasing power output. Each of the cell units is a membrane and electrode assembly (MEA) composed of a hydrogen electrode (anode), an oxygen electrode (cathode), and a proton exchange membrane (PEM) interposed between the electrodes.
The cell unit gains electromotive force via oxidation reaction occurred in the hydrogen electrode and reduction reaction occurred in the oxygen electrode by providing hydrogen to the hydrogen electrode as fuel and providing oxygen or air to the oxygen electrode as an oxidizing agent. A platinum catalyst is used in the electrodes for promoting the electrochemical reactions in the electrodes. Protons (hydrogen ions) generated on the hydrogen electrode moves to the oxygen electrode via the PEM.
However, the fuel cells using PEFC are not suitable as the power supplies for electronic devices such as compact and portable devices because the cells have problems such as requiring upgrading of infrastructure for providing hydrogen gas fuel, and having low energy densities per unit volume of hydrogen gas. In order to overcome such problems, there has been investigated a reforming type fuel cell in which hydrogen is obtained by reforming hydrocarbon fuel or liquid fuel such as methanol. However, such a fuel cell is also not suitable as the power supplies for portable devices and the like because the fuel cell requires a reforming device besides the fuel cell itself.
As a technique applicable to size reduction of fuel cells, a direct methanol fuel cell (DMFC) has been developed that has a power generation cell configuration similar to the PEFC and effects anode oxidation reaction by providing methanol as fuel directly to an anode electrode. The DMFC, where methanol is provided directly to the anode, is an electric power generator operable at ordinary temperature. The DMFC has an advantage of easily achieving its size reduction because methanol fuel is provided directly to the anode and the DMFC does not require a reforming device for extracting hydrogen from liquid fuel. Methanol has high energy density and provides ease of handling.
The DMFC uses as anode fuel a methanol aqueous solution at a concentration of from 3% to 64% by mass. According to an anode equation: CH3OH+H2O→CO2+6H++6e, 1 mole of water is required to 1 mole of methanol, and the concentration of a methanol aqueous solution in this case is 64% by mass. The higher the concentration of methanol fuel is, the further the volume of a cell including a fuel container per energy capacity can be decreased. However, the higher the methanol concentration is, the more pronounced a crossover phenomenon becomes where methanol passes through the proton exchanger to react with oxygen on the cathode side thereby decreasing output voltage. In order to inhibit the phenomenon, an optimum methanol concentration is decided depending on the methanol permeability of a proton exchanger to be used. The cathode equation of the DMFC is 3/2O2+6H++6e→H2O, and the DMFC uses oxygen in air as cathode fuel.
Methods for providing the anode fuel and the cathode fuel to the catalyst layers of electrodes respectively include an active mode of effecting the forced circulation of the fuels by using an auxiliary machine such as a pump; and a passive mode of providing fuels without using auxiliary machines by exposing the catalyst layer to air in the case of gaseous fuel or by using capillary action or gravity drop in the case of liquid fuel. The active mode has advantages such as enabling high power output by providing air, and enabling use of high concentration methanol fuel by circulating water produced at the cathode, whereas the mode has disadvantages of the difficulty of achieving size reduction because an auxiliary machine is required for providing fuel, and electric power consumption by the auxiliary machine. The passive mode has an advantage of being suitable for achieving size reduction, whereas the mode has a disadvantage of being difficult to obtain high power output because providing fuel tends to become a rate-determining step.
One cell unit of a fuel cell has low output voltage, and a plurality of cell units is stacked electrically in series according to required voltage for load. The methods of stacking cell units include plane stacking of arranging the cell units on a plane; and bipolar stacking of heaping the cell units via a separator the both surfaces of which have fuel channels. The plane stacking is suitable for compact fuel cells for portable devices and the like because the cathode electrodes of all the cell units are exposed on the surfaces and air can be easily taken in whereby thinner fuel cells can be produced.
As mentioned above, the passive type fuel cell is suitable for size reduction whereas providing fuel tends to become a rate-determining step and it is difficult to obtain high power output. In general, a fuel cell with larger electrode area produces larger power output. In order to achieve higher power output in the passive mode, it is effective to employ the plane stacking and making the area of an oxygen electrode as large as possible within the size of portable devices or the like to which the fuel cell is mounted. Therefore, a power supply suitable for mounting on compact and portable electronic devices and the like is a passive-mode direct methanol fuel cell in which cell units are stacked in the plane.
When such a fuel cell has a membrane and electrode assembly with large area which assembly comprises a proton exchange membrane, a hydrogen electrode, and an oxygen electrode which membrane is interposed between the electrodes, shape irregularities such as warp or waviness tend to occur on the sheet-like membrane and electrode assembly because of low processing accuracy, and which results in problems such as being incapable of achieving uniform contact between the membrane and electrode assembly and a current collector. As a result, fuel cells with large areas have a problem of low current collecting efficiency, which means that the ratio of electric power that can be extracted via a current collecting plate with respect to electric power actually produced in the membrane and electrode assembly becomes low. In order to achieve uniform contact between the membrane and electrode assembly and the current collector, it is necessary to apply excessively large pressing force of the current collector against the membrane and electrode assembly or to control the distribution of the pressing force. The proton exchange membrane, the hydrogen electrode, and the oxygen electrode which electrodes sandwich the membrane are often formed as a single-piece configuration: MEA. In order to achieve uniform contact between the MEA and a current collector with which electric power is extracted from the electrodes, it is preferred that the casing contains the MEA and the current collector, and pressing force is applied to the MEA and the current collector by the casing.
Such a fuel cell is, for example, described in Patent Document 1, and FIG. 6 is the exploded perspective view of the fuel cell. This fuel cell card is formed as a PC-card-sized functional card by stacking seven main plate-like components. The fuel cell card is mainly composed of, in the order from top to bottom, an upper casing 214 of the fuel cell; an upper current collector 216 on the oxygen side; a pair of membrane and electrode assemblies 211 and 211 provided at upper position from the center of the fuel cell; a hydrogen supplying part 213 for supplying hydrogen as fuel gas which part is positioned at the center of the fuel cell; a pair of membrane and electrode assemblies 212 and 212 provided at lower position from the center of the fuel cell; a lower current collector 217 on the oxygen side; and a lower casing 215 paired with the upper casing 214 to form the casing of the fuel cell. To the fuel cell card, a hydrogen occluded stick 218 can be connected. The stick is a plate having almost the same thickness as the fuel cell card and can supply hydrogen. Hydrogen is supplied from a rod-like projection 220 formed on the connecting side of the hydrogen occluded stick 218.
The fuel cell card is formed to have rounded corners for providing good portability. The card is configured so that the plate-like upper casing 214 is combined with the lower casing 215, and the upper casing 214 is fixed to the lower casing 215 by using screws or the like (not shown). The upper casing 214 has a plurality of openings 231 as a gas inlet for introducing oxygen into the casing. In this example, each of the openings 231 is a nearly rectangle through-hole, and two units of 15 holes arranged in 5 lines and 3 rows are formed side by side. The upper casing 214 has 30 openings 231 in total. Through the openings 231, the electrode on the oxygen side is exposed to air as mentioned later, whereby oxygen is introduced sufficiently without additional air intake devices. Water is produced from the membrane and electrode assemblies 211 and 212 on the production of electromotive force. But water produced on the surfaces of the electrodes can be removed sufficiently because the openings 311, 231, 261, and 241 are opened widely and the assemblies are exposed to air.
The upper casing 214 and the lower casing 215 may be made of metallic material such as stainless steel, iron, aluminum, titanium, or magnesium; resin material excellent in heat resistance and chemical resistance such as epoxy resin, ABS, polystyrene, PET, or polycarbonate; or composite material such as fiber reinforced resin. The plate portions of the upper casing 214 and the lower casing 215 have openings 231 and 241, which are rectangle cutouts, formed in two units of holes arranged in 5 lines and 3 rows.
Patent Document 1: JP-A-2003-86207 (paragraphs 0048 and 0061, FIGS. 5 and 8)
In the DMFC, a phenomenon called crossover occurs where unreacted methanol passes through an electrolytic membrane to reach the cathode. The methanol leaking to the cathode side in the crossover is directly subjected to electrooxidation to produce formic acid as a by-product. The formic acid is a highly corrosive by-product voluntarily dissolving in water. The formic acid dissolves in water produced on the production of electromotive force to produce an aqueous formic acid solution. The aqueous formic acid solution is ejected from the system via the openings 311, 261, 231, and 241 of the upper current collector 216, the lower current collector 217, the upper casing 214, and the lower casing 215. In the course of the ejection, the aqueous formic acid solution contacts the upper current collector 216, the lower current collector 217, the upper casing 214, and the lower casing 215, and can corrode these parts.
The Patent Document 1 discloses preparing the casing by using metallic material or resin material excellent in heat resistance and chemical resistance. A casing made of metallic material obviously causes the problem of corrosion. A casing made of resin material overcomes the problem of corrosion caused by formic acid. However, a casing is required to have high mechanical strength to apply pressing force to an MEA and a current collector for the purpose of achieving uniform contact between the MEA and the current collector. In order to obtain the strength by using resin material, the thickness of a casing has to be increased. As a result, a casing made of resin material has large thickness and heavy weight, and such a casing is difficult to employ for a power supply mounted on compact and portable electronic devices and the like.
An object of the present invention is, therefore, to provide a casing containing an MEA which casing is sufficiently corrosion resistant to formic acid produced by the electrode reaction of the MEA. Another object of the present invention is to provide a casing made of material having a specific gravity as small as possible which casing can apply appropriate pressing force to an MEA and a current collector without increasing the thickness of the casing and which casing is suitable for a power supply mounted on compact and portable electronic devices and the like.